The significance of line width control in mask production has become greater with decreasing tolerable defect size. An exemplary defect that adversely affects the quality of a reticle is illustrated schematically in FIG. 1. Methods used for controlling line width dimensions in masks for sub micron devices are known in the art. Thus, by way of example, U.S. Pat. No. 4,392,120 to Kikuo et al. discloses a system, for inspecting a pattern formed on a base such as a photo-mask, by means of laser beam scanning, which includes a device for detecting the body and edges of the pattern. A memory device having a plurality of memory units for separately storing the detected body and plurality of edges of the pattern, a device for measuring the width of the body of the pattern between two parallel edges of the pattern, a device for detecting and correcting missing pattern edges, a device for inverting the pattern, a device for reducing the pattern and a device for eliminating pinholes and stains within a pattern. The system of Kikuo et al. has several significant shortcomings. For one, it enables to detect edges only in predetermined orientations. As is clearly shown in FIG. 5 of the Kikuo et al. patent, in order to measure the width between edges, data obtained on the entire relevant environment must be stored. Storing the entire environment, at the required resolution poses undue constraints on the processor which must operate in real time. Thus, in order to reduce the volume of data that should be processed in real-time, the Kikuo et al. patent produces images of low resolution, which of course adversely affects the accuracy of the resulting line width measurements.
Another reticle inspection system is disclosed in U.S. Pat. No. 5,563,702, and the basic concept of inspection is based on line width measurement device as described in columns 15 and 16 of the '702 patent. The system of the '702 patent share significant shortcomings of the hitherto known devices, as described above.
Accordingly, there is a need in the art for a new detector which affords reticle quality inspection based on revealing line width errors. There is yet further need in the art for a detector capable of measuring in real time and in high accuracy, short and long lines, which the case may be. The detector may be incorporated e.g. into a die-to-die, die-to-database inspection systems and others.